1. Field of the Invention
The invention relates to a suspension system of a vehicle, and more particularly to a suspension system which can control roll stiffness distribution between a front-wheel side and a rear-wheel side.
2. Description of Related Art
Each of Japanese Patent Application Publications No. JP-A-7-186674, No. JP-A-7-32845, and No. JP-A-5-319055 describes a suspension system which can control roll stiffness distribution. For example, the suspension system described in Japanese Patent Application Publication No. JP-A-7-186674 includes a front roll stiffness changing device; a rear roll stiffness changing device; a roll moment distribution estimating device; and a roll stiffness distribution control device. The front roll stiffness changing device changes roll stiffness on the front-wheel side of a vehicle. The rear roll stiffness changing device changes roll stiffness on the rear-wheel side. The roll moment distribution estimating device estimates distribution of a roll moment that is caused in the vehicle between the front-wheel side and the rear-wheel side of the vehicle. The roll stiffness distribution control device controls the front roll stiffness changing device and the rear roll stiffness changing device so as to distribute roll stiffness between the front-wheel side and the rear-wheel side according to the result of estimation made by the roll moment distribution estimating device.
In such a suspension system that can control roll stiffness distribution, if both of actuators of the front roll stiffness changing device and the rear roll stiffness changing device have sufficient driving performance, the roll stiffness distribution of the vehicle can be controlled as intended.
However, at least one of the actuators of the front roll stiffness changing device and the rear roll stiffness changing device may not have sufficient driving performance. For example, there may be the case where size of at least one of the actuators cannot be increased taking into account an installation space, a permissible weight, and the like. Also, there may be the case where the front roll stiffness changing device and rear roll stiffness changing device are the same, and therefore the driving performance of one of the actuators becomes insufficient. Also, the operation amount of the roll stiffness changing device may become insufficient due to insufficient response of the actuator.
For example, when the roll stiff changing device includes a stabilizer bar and an actuator that actively changes the torsion angle thereof, the stabilizer bar may be twisted by a roll moment before the stabilizer bar is twisted by an appropriate angle by the actuator, and the actuator may not be operated because great resistance torque has already been generated. If the speed at which the roll moment increases is low, the actuator can be operated by a predetermined operation amount. However, if the speed at which the roll moment increases is high, the actuator cannot be operated by the predetermined operation amount.
If the operation amount of the front roll stiffness changing device is insufficient, the roll stiffness on the rear-wheel side increases, and the vehicle steer characteristic shifts from a standard steer characteristic, which is a designed steer characteristic, to an oversteer characteristic (the standard steer characteristic is not necessarily a neutral steer characteristic). If the operation amount of the rear roll stiffness changing device is insufficient, the roll stiffness on the front-wheel side increases, and the vehicle steer characteristic shifts to an understeer characteristic.